Process for improved sintering



United States Patent 3,326,679 PROCESS FOR IMPROVED SINTERING lLarrce Wallace, Brooklyn, N.Y., assignor to Alloys Research & Manufacturing Corporation, Woodside, N.Y., a coruoration of Delaware No Drawing. Filed Mar. 12, 1965, Ser. No. 439,436 1 Claim. (Cl. 75--212) This invention relates to an improved method of sintering aluminous compacts prepared by compressing alumi mum-based powders.

The techniques of aluminum powder metallurgy are becoming more wide spread as efforts increase to find new uses for aluminum and new methods of lowering the cost of fabrication. There have :been many suggestions made and many aluminum products produced through powder metallurgy. While there are various different methods of obtaining optimum properties through aluminum powder metallurgy, there are certain basic steps which are generally followed.

Aluminum-based powders, generally containing at least 50% particulate aluminum with the balance of the metallic constituents, if any, being of metals capable of alloying with aluminum, such as copper, zinc, magnesium, manganese, tin, molybdenum, and the like are mixed and shaped. usually under pressure, to the desired configuration. The shaped and compressed powdered metal, generally referred to as a compact, i.e., a partially agglomerated mass, which is generally self-sustaining absent appreciable impact, is then subjected to a sintering operation in a sintering furnace maintained at a temperature required for the particular composition and process. Depending upon the process used, this temperature can range anywhere from about 400 C. up to, if not slightly above the melting point of aluminum. Heretofore, the output of the process has been severely limited by the capacity of the sintering furnace. The compacting operation is relatively rapid but the sintering times involved can range anywhere from minutes to hours. It has not, however, been possible to load the sintering furnaces to any great extent, in order to expedite the process, since during sintering any objects being sintered which are in contact with each other would invariably bond one to the other in much the same fashion as the individual particles within each compact bond one to the other.

The sintering furnace employed can be a batch furnace in which a plurality of compacts are loaded together or it can be a continuously moving belt furnace. In any case, care has to be taken to insure that only one layer of the compacts is on each shelf or belt of the furnace and that adequate clearance exists between adjacent compacts. These precautions require a large expenditure and often serve to render powder metallurgical fabrication noncompetitive with other methods for the production of a given product.

It has now been found that if a coating is applied to compacts before sintering and if this coating is of a material that is non-reactive with aluminum and nonbondable to aluminum at the sintering temperatures, this coating will serve to permit the random loading of the compacts including the stacking of compacts one to the other and the touching of adjacent compacts. As a result, by means of this invention, compacts can merely be thrown randomly into the furnace and sintered, thereby eliminating all of the costly positioning heretofore found necessary.

The preferred method of applying the coating to the compact is the application of a suspension of powdered coating material in a liquid vehicle. The liquid vehicle should be one that volatilizes at a temperature below that of the sintering operation, preferably, at a temperature between about 75 C. and 400 C. Preferably, the liquid Fee should also be capable of wetting aluminum and should be chemically inert toward aluminum or any of the other metallic components present, below its volatilization temperature.

Petroleum hydrocarbons boiling within the range of 50 C. to 400 C. are satisfactory for these purposes. The kerosene series of petroleum hydrocarbons are particularly useful in this regard. Water and acetone are other liquids which can be used with success.

The preferred coating materials are graphite and molybdenum disulfide, each of which can easily be dispersed in suitable liquids and applied to the compacts as by brushing, spraying, dipping the compacts into the liquid, or similar methods. The particle sizes of the coating materials are not overly critical. They should be smallsized, generally in the vicinity of from about 100 mesh to about 500 mesh so as to form a reasonably optically continuous coating on the compacts. The use of a liquid vehicle as opposed to applying the powdered coating material directly in the solid state is to prevent any appreciable penetration of the powdered coating material into the compact which might result in affecting the sintering characteristics obtained. When a liquid vehicle is employed, the liquid penetrates any surface pores, thereby preventing penetration by the coating particles. The liquid is easily removed by heating the treated compact at a temperature, below the sintering temperature, to volatilize the liquid vehicle. The concentration of the coating material in the liquid vehicle is not critical but depends instead upon the nature of the respective constituents since the coating suspension should have a viscosity favorable for coating under the conditions employed. It will be understood that optimum concentrations can easily he arrived at. Generally speaking, concentrations of from about 5 to 25 parts by weight of powdered coating material per 100 parts by weight of the liquid vehicle will be adequate.

The use of graphite is desirable since this material is a useful lubricant for facilitating subsequent working operations such as hot or cold working and hence need not be removed after the sintering step, although if no working operations are contemplated, it can, of course, be removed by any convenient method, including washing with a suitable liquid such as water, which can contain a suitable detergent if desired, or by physical abrasion. Where the subsequent lubricating properties of the coating material are not necessary, other coating materials that are chemically inert and non-bondable to aluminum and the other metallic components under the conditions of processing can be used, such as molybdenum disulfide, aluminum oxide, magnesium oxide, and silica.

It has been found that sintered products obtained by means of this invention have the same physical properties after sintering as products not treated by means of this invention. However, the cost of processing is greatly reduced.

Additional details on the method of this invention can be had from the following examples, which, in the opinion of the inventor, represent the best mode of carrying out the invention.

Example I A plurality of compacts thick and ranging in outside diameter from 0.406" to 0.750" were prepared by mixing aluminum powder with 4 weight percent copper, 0.3 weight percent magnesium, weight percent of a hydrocarbon lubricant and compacted to of theoretical density. These compacts were then dipped in a suspension of graphite in water containing 10% by weight of powdered graphite of a paticle size of approximately -325 mesh. The dipped compacts were then allowed to dry in air for three minutes, were placed in an oven and a dried for five minutes in air at 250 F. The coated samples were then dumped at random in a 3%" X 7 tray to an average height of 2%. The tray and its contents were then subjected to sintering in an atmosphere of dissociated ammonia at an average sintering temperature within the furnace of 1800 F. and a dew point of 30 F. The same operation was repeated without dipping the compacts into the graphite solution. It was found that the graphite coated compacts sintered satisfactorily to yield products having the desired physical characteristics and no sticking together of adjacent compacts was observed. The uncoated compacts were observed to have stuck together severely in random fashion, and the adjacent compacts required considerable force to break apart the compacts, in many instances, injuring the surfaces thus broken apart.

Example II Similar results were obtained when the coating material was a suspension of 10 Weight percent molybdenum disulfide in acetone.

Example III Similar results are obtained when suspensions containing, respectively, 10 weight percent of 325 mesh silica in kerosene, 10 weight percent 325 mesh magnesia in kerosene, and 10 weight percent 325 mesh alumina in kerosene are employed. In these instances, a drying operation is not essential since the viscosity of the kerosene is such as to generally keep the coating in place. The compacts are merely dipped into the coating medium, allowed to drain free of excess coating medium, and loaded directly into the sintering furnace to a height of about 1". The sintering temperature within the furnace is adjusted to 1660 F. and no sticking is observed at the conclusion of sintering when the coating is employed, but severe sticking is observed when the coating is not employed.

Having thus described the invention, that which is claimed is:

In a method of producing aluminum-based products by powder metallurgical techniques which comprises shaping a powder mixture of the metallic components under sufficient pressure to form a porous green compact whose porosity corresponds to at least about 95% theoretical density, and sintering the compact, the improvement which permits more efiicient utilization of the sintering facilities by permitting non-adhering contact between compacts during sintering, said improvement comprising applying to the surface of the compacts before sintering a suspension in a volatile vehicle of finely divided particles of a material that is nonreactive with aluminum and non-bondable to aluminum at the sintering temperature, whereby the particles of non-reactive material are retained during and after sintering at the surface of the compacts without substantial penetration of the mass of the compacts.

References Cited UNITED STATES PATENTS 2,076,793 4/1937 Salender 148-12.1 2,132,557 10/1938 Bobrov 148--13.1 2,308,070 1/1943 Frey 148-22 X 2,506,364 5/1950 Tarvie 14813.1 2,641,556 1/1953 Robinson 14827 X 2,941,280 6/1960 Heuer 29-470.9 3,250,838 8/1964 Bartoszak -212 3,254,401 6/1966 Dalton 29424 3,265,600 8/1966 Carter 14827 X FOREIGN PATENTS 227,3 69 3/1960 Australia.

473,178 4/1951 Canada. 1,186,640 2/1965 Germany.

693,114 6/1953 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

A. J. STEINER, Assistant Examiner. 

